How do I overlay a lightmap image on another texture?
A solution is to combine the baked AO texture with your texture using nodes.




Make a copy of your initial object (on the picture the original is white on the left and the copy is yellow on the right)
Assign a new material to the copy
Select the copy and open the node editor (bottom part here)
Add two textures : the baked one and yours (here i just used a full yellow texture)
Add a MixRGB color operator and set it to "multiply"
Connect them all as done on the picture


The result is shown on the top (in rendered mode)

Edit :

An alternative/better way in order to be closer to the original colors. It is using darken (MixRGB node) with a grey color (0.250). The darken effect is moderated by the inverted AOBacked texture.



Edit : for Blender Internal Render (same principle)